The chemical machining or sculpturing of glass articles had its genesis in U.S. Pat. No. 2,628,160. That patent discloses the preparation of photosensitively opacifiable glass articles which, upon being sequentially subjected to ultraviolet radiation and heat treatment, will develop crystals in situ, the identity of those crystals being such as to be much more readily soluble in aqueous hydrofluoric acid than the glass. In practice, the portions of the glass body that are desired to be etched away are exposed to ultraviolet radiation and the rest of the body is masked from the radiation. After exposure, the glass body is heated to a temperature in the vicinity of the softening point thereof to cause the formation of crystallites selected from the group of lithium silicate, barium disilicate, and an alkali metal fluoride. The crystals are many times more rapidly soluble in dilute hydrofluoric acid than is the residual glass.
U.S. Pat. No. 2,971,853 describes the production of glass-ceramic articles which can be chemically machined or sculptured. Such articles are produced from photothermally opacifiable precursor glasses which consist essentially, expressed in weight percent on the oxide basis, of 5.5-15% Li.sub.2 O, 2-25% Al.sub.2 O.sub.3, the ratio Al.sub.2 O.sub.3 :Li.sub.2 O being less than about 1.7:1, 60-85% SiO.sub.2, and a photosensitive metal in the indicated proportions selected from the group of 0.001-0.03% gold, computed as Au, 0.001-0.3% silver, computed as AgCl, and 0.001-1% copper, computed as Cu.sub.2 O, the sum of those components constituting at least 90% of the total composition.
The method for preparing a chemically sculptured, glass-ceramic article contemplates the following eight general steps:
(a) a batch falling within the above composition ranges is melted;
(b) the melt is simultaneously cooled to a glass and an article of a desired geometry shaped therefrom;
(c) the glass article is subjected to ultraviolet radiation to form a latent image therein; those portions of the article which are destined to be chemically removed are exposed to the radiation and remainder of the article is masked therefrom;
(d) the article is heated to a temperature between the annealing point and softening point of the glass for a time sufficient to opacify the glass via the formation of siliceous crystallites therein, including lithium metasilicate;
(e) the article is contacted with an aqueous hydrofluoric acid solution to etch away the lithium metasilicate crystals;
(f) the article is re-exposed to ultraviolet radiation;
(g) the article is reheated to a temperature between the annealing point and softening point of the glass for a time sufficient to opacify the glass via the formation of siliceous crystallites therein, including lithium metasilicate, in an amount sufficient to provide a crystalline structure which is rigid enough to prevent distortion of the article upon heating to the softening point of the original glass; and, finally,
(h) the article is heated to a temperature above the softening point of the original glass and above 800.degree. C., but not above 950.degree. C., to cause further crystallization, including conversion of lithium metasilicate to a lithium-containing silicate having a lower weight percentage of lithium therein.
As can be observed from the above outline of method steps, the formation of a chemically sculptured glass-ceramic article requires a re-exposure to ultraviolet radiation and a second high temperature heat treatment. This latter heat treatment results in a highly crystalline body, the properties of which are more closely akin to the crystal phase present therein than to those of the parent glass. Moreover, the small amount of residual glass in the body will have a greatly different composition from that of the parent glass since the elements making up the crystals will have been removed therefrom. One very important mechanical advantage customarily possessed by a glass-ceramic with respect to the precursor glass therefor is a substantial improvement in strength. Also, the glass-ceramic has a higher use temperature than its parent glass.
It will be appreciated that where no chemical machining is undertaken, steps (d)-(f) will be omitted resulting in a single exposure to ultraviolet radiation and subsequent heat treatments.
Corning Glass Works, Corning, New York, has marketed products made in accordance within the disclosure of U.S. Pat. No. 2,971,853, supra, under the designation Code 8603. That product has the overall approximate composition recited below in terms of weight percent on the oxide basis, the photosensitive metals gold and silver being reported in elemental form:
SiO.sub.2 : 79.9 PA1 Li.sub.2 O: 9.4 PA1 K.sub.2 O: 4.0 PA1 Al.sub.2 O.sub.3 : 3.9 PA1 Na.sub.2 O: 1.5 PA1 ZnO: 1.0 PA1 Sb.sub.2 O.sub.3 : 0.4 PA1 CeO.sub.2 : 0.012 PA1 Ag: 0.12 PA1 Au: 0.0013.
Code 8603 articles can be chemically machined to fine tolerances to yield products of widely-varying conformations and configurations, thereby recommending their utility in such diverse applications as intaglio printing plates and spinnerets for the extrusion of synthetic fibers. In the information storage field, head pads or sliders which support the ferrite element that senses the data magnetically stored on information discs have been fashioned from Code 8063 material. A rather recent development in the design of such units was the introduction in 1973 of the Winchester Head by International Business Machines Corporation, Armonk, New York. This head has a geometry requiring close tolerances and the chemical machinability of Code 8603 has proven to be particularly useful in the fibrication of components therefor. A description of the Winchester Head is provided by Robert M. White in "Disk-Storage Technology", Scientific American, 243, No. 2, pages 142-143, August, 1980.
The storage discs consist of an aluminum metal substrate to which very finely-divided iron oxide particles are bonded utilizing an organic bond. Organic binders presently used include such thermoplastics as vinyl chloride/vinyl acetate copolymers, vinylidene chloride/acrylonitrile copolymers, butadiene/styrene rubber polymers, polyurethane elastomers, and epoxies. Also, some formulations include organic antistatic agents, stabilizers, and lubricants.
Unfortunately, considerable sticking has been experienced between the Code 8063 head pad and the coated aluminum storage disc. Examinations of the surfaces of the head pad and the storage disc have ascertained the presence of a substance on the disc surface believed to be the product of a reaction between the organic components of the disc and the humidity of the surrounding environment. No evidence has been found suggesting that the source of the reaction product is the Code 8063 material. In any event, the substance causes sticking between the head pad and storage disc which seriously impairs the operability of the mechanism. Yet, because of the ease of fabrication of the complex shapes required for head pads from Code 8603 material, an extensive search has been undertaken to devise means for preventing the sticking problem.